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Optimize vacuuming of relations with no indexes.

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If there are no indexes on a relation, items can be marked LP_UNUSED
instead of LP_DEAD when pruning. This significantly reduces WAL
volume, since we no longer need to emit one WAL record for pruning
and a second to change the LP_DEAD line pointers thus created to
LP_UNUSED.

Melanie Plageman, reviewed by Andres Freund, Peter Geoghegan, and me

Discussion: https://postgr.es/m/CAAKRu_bgvb_k0gKOXWzNKWHt560R0smrGe3E8zewKPs8fiMKkw%40mail.gmail.com
This commit is contained in:
Robert Haas
2024-01-18 10:03:42 -05:00
parent 7b1dbf0a8d
commit c120550edb
3 changed files with 108 additions and 102 deletions
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80
src/backend/access/heap/pruneheap.c
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@ -35,6 +35,8 @@ typedef struct
/* tuple visibility test, initialized for the relation */
GlobalVisState *vistest;
/* whether or not dead items can be set LP_UNUSED during pruning */
bool mark_unused_now;
TransactionId new_prune_xid; /* new prune hint value for page */
TransactionId snapshotConflictHorizon; /* latest xid removed */
@ -67,6 +69,7 @@ static void heap_prune_record_prunable(PruneState *prstate, TransactionId xid);
static void heap_prune_record_redirect(PruneState *prstate,
OffsetNumber offnum, OffsetNumber rdoffnum);
static void heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum);
static void heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum);
static void page_verify_redirects(Page page);
@ -148,7 +151,13 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
{
PruneResult presult;
heap_page_prune(relation, buffer, vistest, &presult, NULL);
/*
* For now, pass mark_unused_now as false regardless of whether or
* not the relation has indexes, since we cannot safely determine
* that during on-access pruning with the current implementation.
*/
heap_page_prune(relation, buffer, vistest, false,
&presult, NULL);
/*
* Report the number of tuples reclaimed to pgstats. This is
@ -193,6 +202,9 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
* (see heap_prune_satisfies_vacuum and
* HeapTupleSatisfiesVacuum).
*
* mark_unused_now indicates whether or not dead items can be set LP_UNUSED during
* pruning.
*
* off_loc is the offset location required by the caller to use in error
* callback.
*
@ -203,6 +215,7 @@ heap_page_prune_opt(Relation relation, Buffer buffer)
void
heap_page_prune(Relation relation, Buffer buffer,
GlobalVisState *vistest,
bool mark_unused_now,
PruneResult *presult,
OffsetNumber *off_loc)
{
@ -227,6 +240,7 @@ heap_page_prune(Relation relation, Buffer buffer,
prstate.new_prune_xid = InvalidTransactionId;
prstate.rel = relation;
prstate.vistest = vistest;
prstate.mark_unused_now = mark_unused_now;
prstate.snapshotConflictHorizon = InvalidTransactionId;
prstate.nredirected = prstate.ndead = prstate.nunused = 0;
memset(prstate.marked, 0, sizeof(prstate.marked));
@ -306,9 +320,9 @@ heap_page_prune(Relation relation, Buffer buffer,
if (off_loc)
*off_loc = offnum;
/* Nothing to do if slot is empty or already dead */
/* Nothing to do if slot is empty */
itemid = PageGetItemId(page, offnum);
if (!ItemIdIsUsed(itemid) || ItemIdIsDead(itemid))
if (!ItemIdIsUsed(itemid))
continue;
/* Process this item or chain of items */
@ -581,7 +595,17 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum,
* function.)
*/
if (ItemIdIsDead(lp))
{
/*
* If the caller set mark_unused_now true, we can set dead line
* pointers LP_UNUSED now. We don't increment ndeleted here since
* the LP was already marked dead.
*/
if (unlikely(prstate->mark_unused_now))
heap_prune_record_unused(prstate, offnum);
break;
}
Assert(ItemIdIsNormal(lp));
htup = (HeapTupleHeader) PageGetItem(dp, lp);
@ -715,7 +739,7 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum,
* redirect the root to the correct chain member.
*/
if (i >= nchain)
heap_prune_record_dead(prstate, rootoffnum);
heap_prune_record_dead_or_unused(prstate, rootoffnum);
else
heap_prune_record_redirect(prstate, rootoffnum, chainitems[i]);
}
@ -726,9 +750,9 @@ heap_prune_chain(Buffer buffer, OffsetNumber rootoffnum,
* item. This can happen if the loop in heap_page_prune caused us to
* visit the dead successor of a redirect item before visiting the
* redirect item. We can clean up by setting the redirect item to
* DEAD state.
* DEAD state or LP_UNUSED if the caller indicated.
*/
heap_prune_record_dead(prstate, rootoffnum);
heap_prune_record_dead_or_unused(prstate, rootoffnum);
}
return ndeleted;
@ -774,6 +798,27 @@ heap_prune_record_dead(PruneState *prstate, OffsetNumber offnum)
prstate->marked[offnum] = true;
}
/*
* Depending on whether or not the caller set mark_unused_now to true, record that a
* line pointer should be marked LP_DEAD or LP_UNUSED. There are other cases in
* which we will mark line pointers LP_UNUSED, but we will not mark line
* pointers LP_DEAD if mark_unused_now is true.
*/
static void
heap_prune_record_dead_or_unused(PruneState *prstate, OffsetNumber offnum)
{
/*
* If the caller set mark_unused_now to true, we can remove dead tuples
* during pruning instead of marking their line pointers dead. Set this
* tuple's line pointer LP_UNUSED. We hint that this option is less
* likely.
*/
if (unlikely(prstate->mark_unused_now))
heap_prune_record_unused(prstate, offnum);
else
heap_prune_record_dead(prstate, offnum);
}
/* Record line pointer to be marked unused */
static void
heap_prune_record_unused(PruneState *prstate, OffsetNumber offnum)
@ -903,13 +948,24 @@ heap_page_prune_execute(Buffer buffer,
#ifdef USE_ASSERT_CHECKING
/*
* Only heap-only tuples can become LP_UNUSED during pruning. They
* don't need to be left in place as LP_DEAD items until VACUUM gets
* around to doing index vacuuming.
* When heap_page_prune() was called, mark_unused_now may have been
* passed as true, which allows would-be LP_DEAD items to be made
* LP_UNUSED instead. This is only possible if the relation has no
* indexes. If there are any dead items, then mark_unused_now was not
* true and every item being marked LP_UNUSED must refer to a
* heap-only tuple.
*/
Assert(ItemIdHasStorage(lp) && ItemIdIsNormal(lp));
htup = (HeapTupleHeader) PageGetItem(page, lp);
Assert(HeapTupleHeaderIsHeapOnly(htup));
if (ndead > 0)
{
Assert(ItemIdHasStorage(lp) && ItemIdIsNormal(lp));
htup = (HeapTupleHeader) PageGetItem(page, lp);
Assert(HeapTupleHeaderIsHeapOnly(htup));
}
else
{
Assert(ItemIdIsUsed(lp));
}
#endif
ItemIdSetUnused(lp);